Access to a blood vessel is generally obtained by introduction of a needle or a catheter into a vein.
With hemodialysis, blood vessel access constitutes one or more needles or catheters, through which blood is taken out to an extracorporeal blood circuit where the treatment occurs. With hemodialysis the blood normally passes through the extracorporeal blood circuit at a relatively high speed, on the order of up to 500 ml/min. Blood is normally taken out through an arterial needle and reintroduced into the body through a venous needle. Hemodialysis using a single needle (single needle dialysis) or catheters also exists.
If the blood vessel access, such as the arterial needle and/or the venous needle, is not placed correctly, malfunctions can occur.
If the arterial needle is positioned too close to the walls of the blood vessel, it can be difficult to achieve sufficient blood flow with the available pump capacity. If the arterial needle is placed outside the blood vessel, the needle will become blocked by the tissues, and no blood flow will be obtained at all. If the arterial needle is outside the body, air will be sucked into the circuit. These conditions are relatively simple to detect in the extracorporeal blood circuit.
If, however, the venous needle is unintentionally loosened, a life-threatening situation can rapidly arise, since the patient can lose a large amount of blood in a short period of time.
With hemodialysis, the dialysis machine is provided with a plurality of detectors which detect dangerous conditions, and which activate clamp devices which stop the extracorporeal blood flow when such dangerous conditions arise.
Normally the dialysis machine is provided with an arterial pressure sensor which measures the pressure in the extracorporeal blood circuit upstream of the circulation pump. An underpressure of between -20 mm Hg and -80 mm Hg is normally present even though levels as low as -200 mm Hg can be produced with large blood flows. If the pressure approaches atmospheric pressure, this indicates that air is being sucked into the system, while an underpressure which is much too low (below about -200 mm Hg) indicates that the arterial needle can be blocked or not properly inserted into the blood vessel or fistula. Other causes can be that the arterial tube is kinked or that the fistula has collapsed due to an incorrect arm position.
The dialysis machine is further provided with a venous pressure sensor downstream of the dialyser, but before the venous needle, normally in connection with a venous drip chamber where the venous pressure is normally between +50 and +150 mm Hg. The pressure can vary depending on the size of the venous needle, variations in the blood flow and the composition of the blood, blocking of the venous needle or the venous blood tubes, or a separate venous blood filter which is often present in the drip chamber. Additional causes can be that the venous needle is unsuitably placed or that the venous tube is kinked. Further causes are changes in the height location of the fistula, for instance if the patient is sitting or lying.
If the venous needle comes out of the fistula, a reduction of pressure at the venous sensor will occur, which can be detected. This detection is, however, rather uncertain. If the tube is moved upwardly through a holder and the end gets stuck higher up than the arm, the pressure in the venous sensor might not be reduced at all, or only reduced insignificantly so that a set alarm level is not reached. Additionally, it may happen that the venous needle comes out when the patient turns, at the same time there being a risk that the patient will lie on the tube so that it is completely or partially blocked, or that the tube will kink.
There is therefore a desire to have a separate means of detecting whether the needle, used in connection with hemodialysis or another extracorporeal blood treatment, is still adequately in position at the blood access site, and in this respect in particular the venous needle.
This problem has previously been solved by providing the venous needle and/or the arterial needle with some form of sensor which detects if the needles move from a predetermined position. One example is to provide the needles with magnets and arrange the sensors on the arm which senses whether said magnets are close to the sensors. Another method would be to provide the arm with a conductivity detector which provides a signal if blood leaks out. The disadvantage with such detectors is that they have to be attached to the patient and simultaneously be electrically connected to the dialysis machine in order to stop the blood pump and disengage the extracorporeal circuit during malfunction.
With catheters for blood vessel access, clogging may occur or the catheter's opening may be located too close to the blood vessel's wall and get stuck due to suction.
European Patent No. 121,931 discloses an apparatus and method for use in a parenteral administration systems for detecting fault conditions. In one embodiment thereof, the fault detection uses high pass filtering of the pressure signals to pass only the signal components attributable to a patient's heartbeat. An alarm signal is thus produced whenever a dropout in the heartbeat pulses is detected.
European Patent No. 332,330 discloses an infusion system for infusing a fluid into a patient comprising an infusion device for delivering the fluid in both a normal delivery pattern and a test pulse, and a conduit for conducting the fluid from the infusion device to the patient. The test pulse creates a pressure wave response in the conduit. Abnormal diffusion can thus be detected by determining the area between a base line and at least a portion of a pressure versus time curve representing the pressure wave respons.